The present invention relates to a control device for controlling the output voltage of a vehicle-mounted generator.
An example of a conventional control device is shown in FIG. 1 in which a generator 1 is driven by an engine (not shown). The generator 1 includes an armature winding 101 and a field winding 102. The control device further includes a full-wave rectifier 2, a voltage regulator 3 and an over-voltage alarm 4.
The full-wave rectifier 2 functions to rectify the a.c. output voltage of the generator 1. The rectifier 2 has a main output terminal B connected to a terminal of a battery 9, an output terminal T connected to the field winding 102, the voltage regulator 3, and the over voltage alarm 4, and another output terminal E is grounded.
The voltage regulator 3 includes a voltage divider composed of series-connected resistors 301 and 302 one end of which is connected to the positive terminal S of the battery 9 to detect the battery voltage at a junction of the resistors. The voltage regulator 3 further includes another voltage divider composed of series-connected resistors 303 and 304 one end of which is connected to a constant voltage source A for supplying a constant voltage thereto when a key switch 5 of the engine is closed to start the latter. At the junction of the series-connected resistors 303 and 304, a fraction of the constant voltage is provided as a reference voltage.
A comparator 305 included in the voltage regulator 3 has an inverted input connected to the junction of the voltage divider 301, 302 and a non-invered input connected to the junction of the voltage divider 303, 304. A Darlington pair 306 of output transistors is connected to the comparator such that it is on-off controlled by the output of the comparator 305. A base resistor 307 is connected to a base of the Darlington pair 306, and a surge absorbing diode is connected in parallel to the field winding 102.
The over-voltage alarm 4 is composed of an output transistor 401 having a base resistor 402, a current limiting resistor 403 connected to the output terminal T of the rectifier 2, a Zener diode 404 connected to a base of a transistor 405, a pair of series-connected resistor 406 and 407 constituting a voltage divider for detecting a voltage of the output terminal T of the rectifier 2, a Zener diode 408, and a transistor 409 which, together with the Zener diode 408, detects an over-voltage.
The rectifier 2, the voltage regulator 3 and the overvoltage alarm 4 are usually mounted on the generator as a unit.
The key switch 5 is connected through a resistor 7 and a reverse current blocking diode 6 to the terminal T for supplying a current to the field winding 102 for an initial energization thereof.
In operation, when the key switch 5 is closed to ignite the engine, an initial energization current flows from the battery 9 through the switch 5, the diode 6 and the resistor 7 to the field winding 102 to make the generator 1 ready to operate. A potential at the output terminal T of the rectifier 2 under this condition is at a relatively low level which is a fraction of the battery voltage due to the presence of impedances of the resistor 7 and the field winding 102. At the same time, a base current is supplied from the battery 9 through the key switch 5 and the base resistor 402 to the output transistor 401 to turn the latter on, so that an alarm lamp 8 is lit.
When the engine starts, the generator 1 starts to generate an a.c. voltage which is full-wave rectified by the rectifier 2, resulting in a d.c. voltage at the output terminal B with which the battery 9 is charged gradually up to the battery voltage. With the gradual charging to the battery voltage, a voltage at the junction of the series-connected resistors 301 and 303 of the voltage regulator 3 increases and, when the voltage at the junction exceeds the reference voltage of the comparator 305, the latter provides a low level output "L", so that the base of the output transistor 306 is grounded to turn itself off to thereby cut the power supply to the field winding 102.
When the power supply to the field winding 102 is cut, the generator 1 stops to generate and, thus, the terminal voltage of the battery 9 and hence the voltage fraction at the junction of the resistors 301 and 302 gradually decrease below the reference voltage of the comparator 305, ultimately, so that the output of the comparator 305 becomes a high level "H", upon which the output transistors 306 are turned on to supply power to the field winding 102 to thereby operate the generator 1. The voltage regulator 3 controls the output voltage of the generator 1 by repeating the above mentioned cycle of operation such that the terminal voltage of the battery 9 becomes constant.
On the other hand, for the over-voltage alarm 4, the voltage at the output terminal T of the rectifier 2 is applied through the current limiting resistor 403 to a cathode of the Zener diode 404. When the voltage at the terminal T exceeds the Zener voltage of the diode 404, the latter and hence the transistor 405 are turned on and the output transistor 401 is turned off to extinguish the alarm lamp 8, indicating that the generator is operating normally.
When a connection of the battery 9 to the output terminal B of the rectifier 2 of the device under operation is broken due to incorrect setting and/or abnormal vibration of the vehicle, the battery 9 is no longer supplied with the output voltage from the generator 1 and, thus, its terminal voltage and hence the voltage at the junction between the resistors 301 and 302 are not increased any more. Therefore, the output of the comparator 305 remains at the high level, holding the conduction state of the output transistors 306. Thus, the voltage control of the voltage regulator 3 becomes impossible and the output voltage of the generator 1 and hence the field current may increase abnormally, resulting in damage to the output transistors 306.
On the other hand, when a connection to terminal S of the battery 9 is broken for similar reasons, the voltage at the junction between the resistors 301 and 302 becomes zero and the output of the comparator 305 is held at the "H" level and the output transistor 306 may be damaged, similarly. If the connection to terminal B is maintained correctly under such condition, an abnormally high voltage may be applied to the battery 9 as well as other electrical loads of the device which are not shown, causing the lives of these components to be shortened. For the over-voltage alarm 4 under these conditions, the voltage divider composed of the series-connected resistors 406 and 407 is supplied with the voltage at the terminal T of the rectifier 2 and, when the voltage at the terminal T reaches an over-voltage detection value set by the Zener voltage of the Zener diode 408 and the dividing ratio of the divider, the Zener diode 408 and hence the transistor 409 are turned on and the transitor 401 is turned off. Thus, the output transitor 401 is turned on to light the alarm lamp 8, indicating that the generator 1 is out of control of the voltage regulator 3. That is, the over-voltage alarm 4 can indicate only an over-voltage condition and there is no protective function provided.
U.S. Pat. No. 4,349,854 discloses a power generation control system including an abnormal voltage detection dircuit which has a function of cutting an output Darlington pair of a voltage regulator off when the B terminal voltage exceeds a set voltage, to stop a power supply to the field winding to thereby stop the power generation.
In this abnormal voltage detection circuit, however, once the Darlington pair is cut off, it can not be turned on again unless the key switch is opened. That is, the abnormal voltage detection circuit disclosed in U.S. Pat. No. 4,349,854 has no function of regulating the generator output.